The present invention relates to equipment for drilling boreholes and particularly equipment for steering a rotary drill string so that the borehole reaches a target objective. In the petroleum industry when a borehole is drilled, it must intercept a particular formation in a particular location. In the case of boreholes drilled on land, the objective may be directly beneath the drilling rig or it may be at some location to the side of the drilling rig. In either case, it is necessary to monitor the progress of the borehole and take necessary steps to correct any deviation of the borehole which would cause it to miss its target objective. In the case of purposely deviated boreholes, it is necessary to continuously monitor the progress of the borehole and take steps to insure the direction of the borehole is such that it will intercept its target objective at the proper depth. One well known method for controlling the inclination of a borehole or straightening a borehole which is tending to become too inclined is by controlling the weight on the drilling bit. If one considers the drill string to be a pendulum, it is easily seen that the weight of the drill string will tend to cause the drill to assume a vertical position. Thus, if one controls the weight on the drilling bit the natural weight of the drill string can cause the borehole to return to a near vertical position. Conversely, it is also true that increasing the weight on the drill bit will cause the borehole to incline at a steeper angle.
While all of the above methods are known for controlling the inclination of the drill bit, and guiding or steering it so that it intercepts its target objective, in the past this has been expensive due to the time involved in making the required measurements. The present practice is to use instruments which can be lowered on a wire line in the borehole to determine the inclination and compass reading of the borehole. Two successive readings allow the determination of the rate of build or drop and the rate of turn. This allows one to estimate the appropriate weight-on-bit and rotary speed for the next interval. The measuring step involves lowering the measuring tool into the borehole, making the measurement, removing the tool and reading the results. Since these steps require considerable time, they are expensive. Thus, the practice is to use a minimum of measurements in an attempt to control or steer the drill string to its objective. This leads to less than optimum trajectory control.
Recently, considerable effort has been devoted to develop measuring while drilling systems to make measurements and transmit the data to the surface while drilling. The two systems that have received the most effort involve transmitting data to the surface using pressure pulses produced in a mud stream and a hard-wired system wherein the data can be transmitted over an electrical circuit to the surface. The first system, while being relatively simple, is limited to very low data rates and only a minimum of information can be transmitted. The second system, while more difficult to develop, provides a fast data rate which is capable of transmitting a considerable amount of data to the surface. In both systems, some means has been incorporated for measuring the inclination and orientation of a borehole and transmitting it to the surface.